The invention relates to plastic draw tape bags and more particularly, to a non-destructive hem seal for such bags.
A draw tape bag includes of two layers of plastic film which are sealed on three sides and open on the remaining side to form an opening in the bag. A hem securing the draw tape is provided at the periphery of the open end whereby the tape is accessed through openings in the hem. By pulling the draw tape, the opening in the bag closes. Consequently, the draw tape serves as a handle whereby the bag may be grasped to be subsequently transported. The hem in a draw tape bag is formed by two layers of film which are fused together to create a hem seal. The hem seal is typically created by heating the film until it melts and then fusing the two layers together. Heat sealing operations typically create a strong bond which cannot be separated without destroying the film, otherwise known as a destructive bond. Draw tape bags and methods for making draw tape bags are shown in U.S. Pat. Nos. 4,867,735, 4,966,059, and 5,006,380 which are incorporated herein by reference.
Under certain circumstances, a draw tape bag with a destructive hem seal may fail if it is filled with waste and lifted by the hem in contrast to being lifted by the draw tape. The draw tape distributes forces more equally around the perimeter of the hem seal. For example, a person trying to remove a draw tape bag from a garbage container will often grab the bag at or near the hem before using the draw tape to close the bag. As a result, the stresses from the weight of the bag and any additional forces needed to remove the bag from the container are concentrated at the portion of the hem seal where the bag is being grasped. Because heat sealing operations may thin or weaken the film at the point of fusion, the film may tear at the seal when the bag is removed from the garbage container. Under such circumstances, the failure of the bag may cause the contents of the bag to escape.
Therefore, a need exists for a bag with a non-destructive seal, or a seal which opens when stressed. Such a bag would allow the hem seal to partially open and relieve the stress at the seal. Thus, such a bag would not tear prematurely at the hem seal and the bag would only fail when the forces on the bag exceed the load bearing limit of the film. More specifically, the hem seal may experience a load during use which may create stress concentrations along segments of the hem seal. The non-destructive hem seal will partially open to relieve the stress at these segments of the hem seal. The non-destructive hem seal will open only to the extent that the load exceeds the bond strength of the seal. Once the film of the bag absorbs the load and the stress concentration is relieved, the hem seal will stop opening and the remaining portion of the seal will remain intact to adequately secure the draw tape. The localized stress relief effect relieves the stress at the stressed segment of the hem seal while the film layers continue to absorb and distribute the load. Thus, the stress concentration is dissipated without tearing the film at or near the hem seal.
A general object of the present invention is to provide a reliable draw tape bag which will not tear at the hem seal before the load bearing properties of the film fails. Another object of the invention is to create a hem seal which will partially open, thus relieving the stress concentrations around the seal, when requisite forces are present. Another object of the invention is to create a non-destructive hem seal in which the film is bonded by an adhesive, heat sealing, or co-extrusion means. A further object of the present invention is to create a bond such that the force required to open the seal is less than the force required to thin the film to the point of tearing. Another object of the invention is to create a bond strong enough to secure the draw tape in the hem under normal use conditions.
A further object of the present invention is to provide a hem seal which provides opportunities for reducing manufacturing costs and improves process variability. Other objects and advantages of the invention will become apparent upon reading the following description and upon reference to the drawings.
A draw tape bag includes two layers of plastic film in which three sides of each layer are sealed. The remaining side forms an opening in the bag for the reception of items, such as, waste. The hems are provided at the periphery of the open end and are created by folding each layer of film over its corresponding surface. The hems enclose a draw tape whereby the tape is accessed through openings in the hem. The hem is formed by two layers of the film to form a channel adjacent a hem seal, whereby the channel secures the draw tape within the hem. The hem seal is bonded by using an adhesive, a hot seal, or a heat sealed coextruded film such that a non-destructive seal is formed.
The non-destructive seal will open without destroying the two layers of film bonded by the hem seal. When adequate forces are present, the hem will peel from the seal to relieve the stress at the seal. As a result, the bag will not tear prematurely at the hem seal and the bag will fail when the forces on the bag exceed the load bearing limit of the film.